As electronic circuits have become more complex, advances have been made in testing them for faults. Automatic test equipment (ATE) systems have been developed to make specified measurements on electronic circuits and to interpret the results to locate and identify failures.
Printed circuit boards (PCB's) have become especially difficult to test because of the number of components and the difficulty of isolating and accessing them. Traditional guided probing techniques become especially problematic, especially in terms of time consumption, when testing PCB's. One attempt to make the testing more efficient is the use of "bed of nails" test fixtures, which gain access to internal nodes of a circuit board simultaneously. However, even these techniques have not completely overcome problems associated with isolating components and testing their interaction. Also, the fixturing is difficult especially for PCB's having surface mounted devices, coatings, or double sided mounting.
A recent alternative to guided probing and bed of nails testing has been the use of thermographic imaging. Infrared energy emitting from an operating PCB under test produces a unique thermographic image which can be compared with the image of a PCB that is known to be in good operation.
A problem with existing infrared testing systems for PCB's is that the thermal images have required subjective interpretation by a skilled technician. These interpretation difficulties have prevented wide use of thermal information for ATE testing.
Another problem with existing infrared red testing systems is that it is difficult to accurately determine the emissivity of the circuit board under test. Emissivity is a measure of a surface's ability to radiate energy, referenced to that of a perfect, or "blackbody" surface. To accurately determine the temperature of an object from measurements of radiated energy, the emissivity of the object's surface must be known. The problem of determining temperature is made more difficult when the object has areas of different emissivity, such as is the case with PCB's where the devices on the PCB may be plastic, ceramic, bare metal or treated metal. Even components of the same type may have different emissivities on different PCB's.
A need exists for an improved, easy to use, method for producing and diagnosing thermographic images.